The present invention is directed to a process for carrying out the hydrogenation in the anthraquinone process for the production of hydrogen peroxide.
It is known in the mentioned anthraquinone process or AO process (see in this regard the collected summarizing presentation in Ullmanns Enzyklopadie der techn. Chemie, 4th newly revised and expanded edition, Vol. 17, pages 697-704), to dissolve an anthraquinone derivative, and the reaction carrier in a solvent or mixture of solvents and to hydrogenate the thus obtained working solution in the presence of a catalyst. Thereby a portion of the anthraquinone derivative is converted into the corresponding anthraquinone derivative. After filtering off the hydrogenation catalyst the working solution is gassed with oxygen or an oxygen containing gas (usually air), whereby the anthraquinone is reformed with formation of hydrogen peroxide.
After extraction of the hydrogen peroxide dissolved in the working solution with water, the working solution can again be supplied to the hydrogenation step. By continuous repetition of the individual process steps there is attained a circular process in which hydrogen peroxide is synthesized from the gases hydrogen and oxygen.
In the hydrogenation the catalyst is generally employed as a fixed bed or as a suspension catalyst. While in reactors with fixed bed catalysts the catalyst is introduced on a carrier, in using suspension catalysts these catalysts are present in the reactor both as carrier catalysts as well as without carrier.
The illustrative forms of these so-called low pressure-hydrogenation reactors described in the literature are mostly stirred vessel reactors in which by intensive stirring both the catalyst is held in suspension and the gas phase is dispersed (Chem. Ing. Tech., Vol. 52, pages 1-7, especially 5, (1980)).
Furthermore, there are known the so-called double tube loop reactors in which the hydrogen is mixed by means of a power jet through which there is produced a revolving flow which should maintain the catalyst in suspension. The loop reactor described in the literature depends on the principle of mixing in the gas phase by means of a jet nozzle whereby the liquid is recycled via a primary cycle.
These known reactors all have the disadvantage that the hydrogen in the liquid is not completely reacted in a single passage and therefore must be recycled.
In German patent 1542089 and related Kabisch U.S. Pat. No. 3,423,176 therefore there is proposed a reactor which is equipped with tubes changing from thin to thick, see the drawing, whereby the upwardly leading narrow tubes should be flowed through with a velocity of 1.5 to 3 m/sec. The entire disclosure of the Kabish U.S. patent is hereby incorporated by reference and relied upon. It was believed that by limiting the velocity to a maximum of 3 m/sec. in the thin tubes so-called grinding and pulverizing forces are avoided.
The repeated tubular contractions, respective widenings, should effect a good mixing of the hydrogen into the liquid phase so that there can be eliminated special gas distributors or gas inlet apparatus.
Precisely with larger tube diameters (&gt;1000 mm), however, there occurs at the tube widenings a phase separation gas/liquid which can place the function of the reactor completely in question through a breaking down of the flow; this was not considered.
According to the description in German patent No. 1542089 (and in the Kabisch U.S. patent) the there described operating conditions and the hoped for good, thorough mixing of gas, liquid, and suspension-catalyst should develop the full activity of the catalyst.
This assumption is based on the proposition that the specific reaction rate speed determining step is in the transition material gas/liquid. However, experiments to increase the catalyst production further (moles of hydroquinone per m.sup.3.h) through a still better gas distribution, i.e. to increase the reaction, led to no success.